1. Field of the Invention
The present invention relates to an active matrix display device and a driving method thereof. The invention particularly relates to a display device having a switching element such as a thin film transistor (hereinafter referred to as a TFT) and a light-emitting element in each pixel, and a driving method thereof. In addition, the invention relates to an electronic apparatus using the display device and a driving method thereof.
2. Description of the Related Art
In recent years, technology to form a TFT has greatly progressed, and application development for an active matrix display device has been promoted. Particularly, since the field effect mobility (also called mobility) of a TFT using a polysilicon film as an active layer is higher than that of a TFT using a conventional amorphous silicon film, high speed operation is possible. Therefore, by using a driver circuit formed by using TFTs over the same substrate as pixels, control of the pixels can be performed. In a display device in which various circuits include TFTs over the same substrate as the pixels, various advantages such as reduction of manufacturing cost, downsizing, increase of yield, reduction of throughput are obtained.
Research has been activated on an active matrix EL display device having an electroluminescence element (hereinafter referred to as an EL element) which is a light-emitting element as a display element included in each pixel of a display device. An EL display device is also called an organic EL display (OELD: Organic EL Display) or an organic light-emitting diode (OLED: Organic Light-Emitting Diode).
In general, since light emission luminance of an EL element has a proportional relation with a current value flowing to the EL element, an EL display device using the EL element as a display element controls light emission luminance with the current value. As a method of a gray scale expression, in a configuration in which the EL element and the TFT (referred to as a driving TFT) are connected in series between two power supply lines, there is a method in which the driving TFT is operated in a saturation region, and a voltage between a gate and source of the driving TFT is changed to control the current value flowing to the EL element. In addition, there is a driving method in which the current value flowing to the EL element is constant, and light emission luminance is controlled by the time when a current flows to the EL element in a predetermined time to express a gray scale (see the following Patent Document 1).
[Patent Document 1] Japanese Patent Laid-Open No. 2001-5426